The object of this research proposal is to determine if there is a deficiency of 1,25-dihydroxycholecalciferol (1,25-DHCC) in hypoparathyroidism, and if so, to assess the causes and pathogenetic role of this deficiency. By measurement of blood calcium, phosphate and parathyroid hormone (PTH) levels and by determination of urinary cyclic AMP response to PTH infusion, patients with hypoparathyroidism will be divided into hormone-deficient, hormone-ineffective and hormone-resistant groups. Measurement of serum 1,25-DHCC level, intestinal calcium absorption and urinary hydroxyproline excretion will be performed in each of these groups before and after the application of two known stimuli to 1,25-DHCC synthesis: (1) chronic intramuscular PTH injections and (2)phosphate depletion by antacids. The responses of the hypoparathyroid groups will be compared to those of age and sex matched normal subjects. In order to properly interpret stimulated alterations of serum 1,25-DHCC level, diurnal variation of this metabolite will be examined in normal subjects. Since many features of hypomagnesemia and end stage renal disease are consistent with an acquired form of PTH-resistant hypoparathyroidism, patients with these disorders will undergo similar evaluation of vitamin D status before and after magnesium replacement and renal transplantation respectively.